This invention relates to a device for determining the distance to a concealed conductive object, and more particularly, it relates to an apparatus for automatically determining the depth measurement of a concealed conductive object, such as an underground pipe or cable, which is radiating an electromagnetic signal.
A number of electronic devices now exist and are commonly used to determine the location, in a horizontal plane, of buried or otherwise concealed electrically conductive structures. One type of such device uses an alternating current which is impressed on the structure, such as a pipe, wire or cable, by direct connection or inductive coupling. This structure can then be located, in the horizontal plane or laterally, above the ground, by use of a suitable horizontal axis pick-up coil and amplifier with an appropriate indicating device, such as a meter or audio transponder. As the receiver antenna or pick-up coil is brought closer to the structure being located, the signal level increases and the position nearest to the structure, i.e. directly above the structure, produces the strongest signal. One such device is described in the November 1965 issue of the BELL LABORATORIES RECORD, which employs two signal receptors or antennae, offset vertically, which sense a signal being radiated by an object. The ratio of the amplitudes of these signals from the two antennae is used to determine the depth of the object. This is accomplished in two steps. First, an amplified signal from the lower of the two antennae is sensed, and by means of a signal level adjustment, a meter is manually adjusted to full scale responsive to the signal. Thereafter, the lower antenna is disconnected, and an amplified signal from the upper antenna is fed to the same meter. The resulting scale deflection is then representative of the ratio of the signals, and indicates the depth of the object, since the meter is calibrated for this purpose. While this device is capable of providing good results, it is somewhat complex to operate in that it requires a number of manual steps. Moreover, inaccuracies can occur due to interference signals which produce inaccuracies in the measurement of the radiated signal. Inaccuracies may also occur because of errors in the manual adjustments, meter readings, and the exact location of the structure.
Another type of device is described in U.S. Pat. No. 3,893,025 and in Great Britian Pat. No. 1,509,914. However, this type of device uses an entirely different principal whereby the signal output from the two antennae are connected together in an electrically subtractive mode. The signal must be initially manually adjusted by an adjustable signal attenuator. The depth of the object is then read directly from a calibrated scale associated with the attenuator, or associated meter. Since this type of device requires an initial manual adjustment, it is subject to human error. Moreover, this type of apparatus is subject to inaccuracies due to interference signals. Finally, this type of apparatus has a non-linear characteristic and is not suitable for digital display.